1. Field
The present disclosure relates generally to fuel tanks and, in particular, to a method and apparatus for reducing the transfer of energy within the interior of a fuel tank.
2. Background
Fuel tanks in aircraft are often integral structures to the aircraft. For example, the wing structure of an aircraft may be sealed. The internal cavities of the sealed wing structure may be used as a fuel tank. These types of wings are also referred to as “wet wings”.
With a wet wing, components, such as fasteners, hoses, tubing, or other components that extend into the wing, are sealed to exclude the outside from the inside or covered to reduce or eliminate the buildup of electrical charge on conductive surfaces. These components can extend through structures within the fuel tank, such as struts and stringers, or into the walls of the fuel tank via holes formed in the walls. In conventional fuel tanks made from metal, components and the holes through which they extend are sealed to reduce leaking or seepage of the fuel tank formed in the wing. In composite structures, the sealing of metal surfaces and the holes that penetrate the structure have three purposes: 1) the reduction in fuel leakage, with respect to fuel leaking out of the tank; 2) other fluids entering or exiting the fuel tank; and 3) coverage of metal components that can have a propensity to accumulate electrical charge.
Components, such as metallic fasteners, may be sealed. Sealant in the form of seal caps may cover these fasteners. A “seal cap” is a structure that covers an end of a metallic component. The metallic component may be a fastener. The end may be the head fastener or the threaded end of the fastener with a nut. The fastener may be for example, a bolt, a screw, or some other type of fastener.
For example, a seal cap may be attached to the end of a fastener that extends into the interior of the fuel tank. This seal cap is configured to provide a seal against the flow of fuel out of the fuel tank. The seal cap may also reduce or eliminate the accumulation of electrical charge on the surface of the exposed fastener.
Seal caps are often comprised of materials that retain sealing properties when submerged in fuel and/or when left dry for different periods of time. For example, metal seal caps are typically used in fuel tanks for aircraft. These types of seal caps typically have aluminum housings that fit over the protruding end of a fastener on the interior of the fuel tank. Sealants may be placed into the seal caps prior to the seal caps being placed on the fastener. The sealant may be in the form of a plastic forming material.
For example, a seal cap may have an interior that is filled with an uncured sealant. This seal cap with the sealant is then pressed into place on the fastener. When in this position, excess sealant extrudes from around the bottom and from a hole in the top of the cap. This sealant may be blended around and onto the exterior of the cap. The sealant is then reacted to form the final sealant material.
Seal caps also may be configured to provide protection against phenomena, such as sparking, that results from electromagnetic events. The electromagnetic current may be current from a lightning strike. With the use of sealants in seal caps, the size of seal caps and the amount of sealant used may be increased to provide additional protection against electromagnetic events.
This increase of internal mass to the seal cap resulting from the sealant filling the internal volume of a seal cap, however, adds additional weight. With the use of seal caps for each of the fasteners in the fuel tank, the increased weight resulting from this use of seal caps attached to fasteners is undesirable.
Therefore, it would be desirable to have a method and apparatus that takes into account at least some of the issues discussed above, as well as possibly other issues.